During the processing of wafers for forming semiconductor chips or radiation detectors, the surface of the wafer may need to be etched to remove imperfections, to provide a smooth surface, or to adjust the thickness of the wafer surface material. Such surface removal may be done mechanically through an action such as lapping, or may be done chemically by etching the surface. Etching may be accomplished by plasma etching, gas etching, or by liquid etching.
In applications such as the preparation of wafers for forming infrared detectors, the after-processing thickness of the wafer surface material may be important to the ultimate use of the devices. For example, after liquid phase epitaxy (LPE) growth, a certain amount of the Mercury/Cadmium/Telluride (MCT) layer must be removed by etching, to achieve a desired cutoff wavelength of radiation absorption. Different target wavelengths require different amounts of etching. Thus, it is desired to "tune" the detectors by adjusting the wavelength.
For the above and other reasons, it is therefore important in many processing operations to accurately control the amount of wafer surface material removed in a lapping or etching operation. In a chemical etch process, the amount of material removed during the etching operation is dependent upon, among other things, the amount of time that the chemical etchant is in contact with the wafer surface material and the etching rate for that etchant. However, in a liquid etch process, the etch rate may not be a constant as a function of time during the etching operation, which makes determining the amount of material removed during the etching operation difficult to predict. For example, when etching MCT with a liquid bromine-methanol solution (Br2/MeOH), the solution is poured over an MCT film for etching the MCT film, the immediate reaction is a rapid etching of the film. Just above the surface of the film, the solution quickly becomes depleted of bromine, and the etching reaction rate decreases, as illustrated in FIG. 1.
Because of this changing etch rate, and the non-uniform film removal rate, determining the endpoint of an etching operation is difficult. After etching, the part must be removed from the etch apparatus, and examined to determine the thickness of the remaining MCT film. If too much film remains, the part must be reinstalled in the etching apparatus, and another etching operation performed, a time consuming operation. However, examination may reveal that the part has been over-etched, leaving too little MCT, and requiring that the part be rejected and scrapped.